Vitamin D and vitamin B12 testing: when it is useful and when it is not

Vitamin D helps the body absorb calcium and supports bone and muscle health. Deficiency can cause rickets in children and osteomalacia in adults, with bone pain and muscle weakness. Skin makes vitamin D from sunlight, but in the UK sunlight is too weak for this from roughly October to early March. People with darker skin, those who cover their skin, older adults, people who are housebound and those with limited sun exposure are at higher risk of low levels.

• Bone pain, muscle weakness or unexplained aches.

• Known or suspected osteoporosis or osteomalacia.

• Conditions affecting absorption, such as coeliac disease or after bariatric surgery.

• Chronic kidney or liver disease, or medicines affecting vitamin D metabolism.

• Pregnancy or breastfeeding where advised.

Laboratories report levels with ranges that classify deficiency, insufficiency and sufficiency. A borderline result in a healthy person is not necessarily a problem. Treatment of deficiency often uses higher doses for a set period, followed by maintenance, guided by a clinician. Very high doses taken without advice can cause harm, including high calcium. Testing after a treatment course is not always needed.

B12 is needed for blood cell formation and nerve function. Deficiency may cause tiredness, breathlessness, pallor, sore tongue, tingling, numbness, balance problems, memory difficulties and low mood. Causes include dietary lack, pernicious anaemia (an autoimmune condition), gut conditions such as Crohn's or coeliac disease, stomach surgery, and long-term use of metformin or proton pump inhibitors.

A serum B12 test is the usual first step, sometimes with additional tests such as methylmalonic acid or intrinsic factor antibodies if results are borderline. Treatment may be injections, especially if there are neurological symptoms or absorption problems, or high-dose tablets in suitable cases. Deficiency with nerve symptoms needs prompt treatment to prevent permanent damage. People following vegan diets should take a B12 supplement or fortified foods.

Taking a modest daily vitamin D supplement as advised by the NHS does not require a test. Taking very large doses based on a borderline result can be harmful. Fatigue is a common reason for requesting vitamin tests, but it usually has other causes such as poor sleep, low mood, thyroid disease or anaemia. A clinician can help decide which tests are appropriate.